1. Field of the Invention
The present invention relates generally to load cells and, more particularly, to a single-point load cell.
2. Description of Related Art
A variety of single-point load cells are found in the prior art. A single-point load cell is a device adapted for mounting beneath a scale platform to measure a weight or load applied to the platform. In contrast to multiple load cells, wherein several load cells are mounted at separate locations beneath the platform, only one single-point load cell is utilized. Thus load is measured at only a single point beneath the scale platform, rather than at several points simultaneously. A desired property of the single-point load cell is that it be insensitive to bending moments produced by random placement of the load anywhere on the scale platform.
A typical single-point load cell provides a surface for mounting a platform, two or more cantilevered beams oriented to bend or strain upon the application of a load to the platform, and two or more bending beam strain gauges provided on the beams for measuring bending or strain within the beams. In the conventional single-point load cell, the bending beam strain gauges are spaced widely apart along the cantilevered beams for measuring strain at multiple points along the beams.
A conventional two-beam single-point load cell is shown in FIG. 1. The two-beam load cell of FIG. 1 includes a load cell block with a generally rectangular opening extending through the block. The opening extends almost the entire height of the load cell block, leaving only a thin connecting plate or beam at the bottom of the block and a second thin connecting plate or beam at the top of the block. Bending beam strain gauges 200 are positioned along inside surfaces of the connecting beams for measuring strain induced in the connecting beams by a load applied to a top surface of the load cell block. As can be seen from FIG. 1, strain gauges 200 are widely separated along the inside surfaces of the connecting beams.
A conventional three-beam single-point load cell is shown in FIG. 3. The three-beam load cell of FIG. 3 is similar to the two-beam load cell of FIG. 2, except that a connecting beam is provided which spans the opening of the load cell block. A plurality of bending beam strain gauges 202 are provided on opposing top and bottom sides of the connecting beam, rather than on the top and bottom connecting plates as in the two-beam load cell of FIG. 2. Alternatively, all strain gauges 202 are provided either on the top or the bottom of the connecting beam to reduce manufacturing costs. However, as with the two-beam load cell of FIG. 2, the strain gauges of the three-beam load cell are spaced widely apart along opposing ends of the connecting beam.
In the load cell designs of FIGS. 1 and 2, the strain on each beam varies substantially, depending upon the location of the load on the platform mounted to the top surface of the load cell. This variation in strain is conventionally referred to as an "eccentric" load effect. The strain gauges are spaced widely apart in an effort to compensate for the effect of an eccentric load. However, eccentric load compensation is difficult to achieve with widely separated gauge locations because of variations inherent in the structure of the load cell block as the result of normal manufacturing tolerances. Often, a secondary means of compensating for eccentric load errors is provided. However, such secondary means render the load cell more complicated and expensive to manufacture and maintain.
The connecting beams of the load cells of FIGS. 1 and 2 act as cantilevered flexures upon the application of a load to the load cell. As such, the connecting beams typically deflect substantially upon the application of a large load. Often, such a large deflection is nonlinear, and the load cell therefore suffers from nonlinearity hysteresis upon the application of the load.
Thermal gradients within the load cell block can effect the performance of the load cell when strain gauges are widely spaced. Such is a particular problem when a source of heat affects only some of the strain gauges or only a portion of the electronic circuit of the load cell.
Finally, it is often desirable or necessary to isolate the strain gauges to protect the gauges from moisture or harsh chemicals. With a plurality of strain gauges widely spaced apart, it is difficult and expensive to adequately isolate each individual strain gauge.